Adenosine is an extracellular messenger generated by all cells in the body. Adenosine itself, substances that act as agonists of adenosine, and substances that antagonize its actions have important clinical applications. In the heart, an organ whose function depends critically on an adequate supply of oxygen, adenosine regulates the balance between oxygen supply (coronary blood flow) and oxygen demand (cardiac work). Adenosine released from working heart cells increases oxygen supply through coronary dilation and decreases oxygen consumption by slowing heart rate and modulating .beta.-adrenergic stimulation. The protective effects of adenosine are particularly important when cardiac oxygen supply is limited, for example, by coronary artery narrowing.
Several recent reviews describe the adenosine system in detail (Belardinelli, L., J. Linden, R. M. Berne [1989] Prog. Cardiovasc. Dis. 32:73-97; Belardinelli, L., A. Pelleg [1990] J. Cardiovasc. Electrophysiol. 1:327-339; Olsson, R. A., J. D. Pearson [1990] Physiol. Rev. 70:761-845). The cardiac adenosine system consists of three processes: (1) mechanisms for adenosine formation; (2) adenosine receptors and proteins that couple them to effectors; and (3) mechanisms for the removal of adenosine. Selective modification of one or more of these systems by means of drugs such as adenosine receptor antagonists and adenosine uptake inhibitors can modify the actions of adenosine for therapeutic benefit.
Adenosine formation increases when oxygen demand exceeds its supply, thereby promoting the degradation of adenosine nucleotides. The degradation of adenylates released from nerve terminals along with neurotransmitters and the degradation of S-adenosylhomocysteine, a byproduct of methylation reactions, are additional sources of adenosine in the heart. Heart muscle and coronary blood vessel cells take up very nearly all the adenosine generated in the heart, reincorporating that adenosine into the cellular nucleotide pool.
At least two types of receptors mediate the actions of adenosine in the heart. A.sub.1 adenosine receptors (A.sub.1 AR) decrease oxygen consumption, for example, by slowing heart rate, and the A.sub.2 adenosine receptors (A.sub.2 AR) increase oxygen supply by causing coronary vasodilation. The actions of adenosine on cardiac cells are either direct (cAMP-independent) or indirect (cAMP-dependent). The direct actions include the negative dromotropic effect on the AV node. Those electrophysiological effects are the basis of adenosine's anti-arrhythmic properties; adenosine is highly effective (&gt;90%) in terminating paroxysmal supraventricular tachycardia (PSVT). Whereas the direct effects of adenosine occur in the absence of agents that act through adenylate cyclase, the indirect effects reflect the inhibition of this enzyme when it is stimulated by agents such as .beta.-adrenergic agonists.
A number of pharmacological studies employing receptor-selective agonists support the idea that A.sub.2 ARs mediate coronary vasodilation. Although endothelial cells contain A.sub.2 ARs and thus could play a role in vasodilation, they are not essential, for adenosine acts on coronary smooth muscle cells, causing them to relax.
When adenosine is used as a drug, its side effects are usually mild, a reflection of its extremely rapid degradation in the body (seconds). The safety of adenosine in the diagnosis and treatment of PSVT is now well established. An important factor which has inhibited the therapeutic development of the adenosine analogues is the ubiquitous nature of adenosine's action on a variety of tissues.
Two kinds of drugs modify the actions of adenosine according to whether they magnify or attenuate the effects of the nucleoside. Inhibitors of the cell membrane nucleoside transporter block the removal of adenosine from the extracellular space, thereby increasing its concentration and intensifying its action. Adenosine uptake blockers also inhibit the nucleoside transport system in human erythrocytes and cardiocyte membranes and potentiate the cardiac actions of adenosine in the dog.
Methylxanthines competitively antagonize the binding of adenosine to both the A.sub.1 AR and the A.sub.2 AR. Certain naturally occurring methylxanthines such as caffeine and theophylline antagonize the cardiovascular effects of adenosine. For example, the administration of adenosine to patients receiving theophylline fails to produce AV block or terminate PSVT. However, those methylxanthines are relatively weak and, more importantly, are nonselective, antagonizing both the electrophysiological and vasodilatory effects of adenosine in laboratory animals and humans. Theophylline also ameliorates the non-cardiac effects of adenosine such as flushing, local pain, and respiratory stimulation.
Synthetic alkylxanthines, e.g., 8-cyclopentyl-1,3-dipropylxanthine (CPX; see U.S. Pat. Nos. 4,364,922 and 4,980,379), are significantly more potent and selective antagonists at the A.sub.1 AR than are theophylline or caffeine.